Films that allow passage of gases at moderate to high transmission rates are often called breathable. The gases most commonly used to demonstrate a film's breathability are water vapor (also referred to herein as moisture vapor or moisture) and oxygen. The moisture vapor transmission test and oxygen transmission test measure the mass or volume of gas transported across the cross-section of a film in a given unit of time at a defined set of environmental conditions. Breathable films can be classified either as microporous films or monolithic films (which are not porous).
A breathable film can be laminated onto a nonwoven substrate to form a vapor-permeable, substantially water-impermeable multilayer article. A vapor-permeable, substantially water-impermeable multilayer article can refer to an article that allows the passage of a gas but substantially does not allow the passage of water.